I. Field of the Invention
This invention relates to a cable shielding tape comprising a metal strip having an adhesive layer of polymeric resinous material tightly adhered to at least one side thereof and a bond control layer of polymeric resinous material strippably adhered to said first adhesive layer. The invention also pertains to jacketed electrical power and communications cables utilizing such shielding tape.
II. Description of the Prior Art
In the art of designing and constructing electrical cables, especially telecommunication cables such as telephone cables, it is known to assemble insulated conductors in a core and surround it by shield and jacket components. A well known telephone cable design of such construction is referred to in the art as "Alpeth" cable. Such cable is described more fully in the F. W. Horn and R. B. Ramsey paper "Bell System Cable Sheaths Problems and Designs" in A.I.E.E. Processings 1951, Volume 70. The shielding tape of "Alpeth" cable is bare aluminum having a thickness of about 8 mils which is usually corrugated transversely prior to being wrapped about the cable core. The corrugation imparts greater flexibility to the cable and permits bending of the cable without wrinkling or rupture of the shielding tape.
The term "shield or shielding tape" as used herein means a relatively thin layer of any metal, bare or coated, which can provide mechanical protection and electrostatic and electromagnetic screening for the conductors in the core of electrical power and communications cables. The term is also referred to in the art as metallic strip, metallic foil, metallic screen, metallic barrier, metallic sheath, metallic tape, metallic foil-plastic laminate, protective sheath, shielding material, i.e. which terms are often used interchangeably.
When telephone cables are installed underground by being buried directly in soil, the outer polyethylene jacket of such cables may be subjected to damage from the rigors of installation, rocks, rodents, lightning, frost and dig-ins. The underlying shielding tapes can thereby be exposed to soil water and the attendant potential for corrosion.
In conventional plastic jacketed telephone cables, i.e., cables in which the outer jacket is a plastic material such as polyethylene, the jacket is not well adhered to the shielding tape of bare metal. The outer plastic jacket is known to slip over the shielding tape and to bunch into shoulders as the cables are pulled through ducts or plowed into trenches. Furthermore, the shielding tape is known to kink or fatigue resulting from bending stresses during installations.
In order to improve the corrosion resistance of the shielding tape of bare metal, a special adhesive polyethylene film may be applied to cover one or both sides of the metallic strip. Such an adhesive polyethylene film is disclosed in U.S. Pat. Nos. 3,233,036 and 3,795,540. The adhesive polyethylene used for this film contains reactive carboxyl groups which have the ability to develop firm adhesion to the metallic strip and also to the overlying polyethylene jacket. A metallic strip, such as aluminum, which is protected by the adhesive polyethylene is highly resistant to corrosion.
When a polyethylene jacket is extruded over the metallic strip coated with the adhesive polyethylene, the heat from the semi-molten polyethylene bonds the coated strip to the jacket, forming a unitized sheath which combines the strength of the metal component with the elongation and fatigue resistance of the polyethylene component. Such cable construction is referred to in the art as "bonded jacket" cable design. If the heat imparted to polyethylene is sufficiently high, the shielding tape would become hot enough so that the overlapped portions of the tape bond together at the seam, thereby forming a tight tube or pipe around the core of the cable. The "bonded jacket" cable with a sealed seam has significantly improved resistance to moisture penetration into the cable core. The cable construction also has been shown to have high mechanical strength necessary to withstand repeated bending of the cable, i.e. kinking and fatigue failures of the shielding tape, resulting from bending stresses during installations. Further, the stresses induced by the temperature cycles under service conditions are reduced substantially.
Although the desired bond between the outer jacket and the shielding tape of bare metal and the prevention of moisture penetration into the cable core are achieved by coating the metal with the adhesive polyethylene, some problems have been encountered in terminating and splicing the cables. More specifically, it is cumbersome to separate the jacket from the shielding tape for the purpose of making electrical connection to the tape. While it is possible to terminate and splice the "bonded jacket" cables without separating the jacket from the shielding tape, it has been shown that the quality of electrical connections is not as good as that with the jacket removed. More particularly, the electrical properties of the connections to the shielding tape are known to change less with time than the connections to the shielding tape and bonded jacket of electrical cables.
To solve the problem of terminating and splicing the "bonded jacket" cables, several approaches have been suggested. U.S. Pat. No. 3,770,570, for example, discloses a shielding tape comprising a metallic strip, such as aluminum, and an adhesive copolymer of ethylene and acrylic acid adhered to at least one side thereof. The resulting structure has zones of reduced adhesion between the metallic strip and the copolymer layer induced by a fugitive release agent or latent adhesive disposed in prescribed areas between the adhesive copolymer layer and the metallic strip. The adhesive copolymer is thereby easily removed from the designated areas to facilitate cable terminations. However, the corrosion resistance of the shielding tape in the main span of the cable, i.e., the span of the cable between two terminations, and the bond strength of the jacket to the shielding tape are sacrificed. Furthermore, the corrosion resistance of the shielding tape at the termination point is poor since the stripping of the adhesive copolymer exposes bare metal in this area.
U.S. Pat. No. 3,379,821, teaches the application of oil, talc, or other foreign substances on the outer surface of the adhesive copolymer, to reduce adhesion between the shielding tape and the jacket. However, the results are not uniform so that some parts of the jacket would be bonded to the copolymer more tenaciously than other parts and, at some locations, there would be little or no bond. Furthermore, such a practice requires an additional operating step in the manufacture of cables. U.S. Pat. No. 3,876,462 discloses a high voltage power cable having an outer semiconductive layer of tough vulcanized polyolefin which is readily strippable from the adjacent insulating layer to facilitate splicing of the cable. The strippability is imparted to the cable by sulfonating the surface of the vulcanized polyolefin layer prior to enclosing it in the semiconductive outer layer. Once again, such practice requires an additional operating step, necessitating the use of expensive equipment in the manufacture of cables.
U.S. Pat. No. 3,424,631 describes yet another approach; namely, a method of preparing cables having a shielding tape of bare metal with a controlled jacket strippability. This method comprises placing circumferentially or longitudinally onto the shielding tape a covering of a film of a copolymer of ethylene and a polar monomer, and applying a polyolefin outer jacket to the ethylene copolymer whereby the polyolefin outer jacket firmly adheres to but is strippable from the shielding tape. The degree of adhesion or strippability of the polyolefin outer jacket to the shielding tape is controlled through variations in the polar comonomer content and/or melt index of the copolymer film. A similar approach is also used in U.S. Pat. No. 3,891,791, which uses various blends of an adhesive copolymer with polyethylene homopolymer. With both of the approaches described above, the adhesive film is separated from the shielding tape at the metal and film interface, leaving the metal unprotected from corrosion after stripping.
U.S. Pat. No. 3,950,605 discloses a shielding tape comprising a metallic strip, such as aluminum, and an upper layer of ethylene and vinyl acetate copolymer containing 3 to 8 weight percent of vinyl acetate and a lower layer of an ethylene and vinyl acetate copolymer containing 10 to 45 weight percent of vinyl acetate. The lower layer is thermally pressed in contact with the metallic strip and the upper layer is capable of being adhered to an outer polyethylene jacket when the jacket is extruded over the shielding tape during the manufacture of the cable. The lower layer has excellent adhesion to the metallic strip, but poor adhesion to polyethylene; on the other hand, the upper layer has excellent adhesion to polyethylene, but poor adhesion to the metallic strip. Hence, the patent teaches the use of composite plastic film to obtain "perfect adhesion" between the metallic strip and the polyethylene jacket. The adhesion between the two layers of composite plastic film is effected by thermal welding; therefore, it is difficult to peel off and separate them. Furthermore, when the jacket is stripped from the cable made with the shielding tape of this patent, a separation will occur as the metal and the lower layer interface, leaving the metal unprotected from corrosion after stripping.
For an optimum control of jacket strippability and greater reduction in bond strength between the jacket and the shielding tape, it may be necessary to combine the above approaches of controlling bond strength with reduced melt temperatures of jacket extrudate. Sometimes lowering the jacketing temperature, by itself, may suffice as a bond control method since bonding of the coating of the shielding tape and jacket is a function of time and temperature. Higher bond strengths usually result when an interface to be bonded is subjected to increasing temperatures and longer time periods above the melting point of the respective materials to be bonded. Therefore, use of lower melt temperatures to achieve strippability is inconsistent with the extrusion conditions needed for high quality jacket and the optimum mechanical reinforcement through bonding of the shielding tape and the jacket.
Accordingly, it is an object of this invention to provide a shielding tape to which an outer jacket may be firmly bonded thereto wherein the jacket may be removed to facilitate the splicing and grounded procedures. Another object of this invention is to provide corrosion protection in all areas of such shielding tape by allowing removal of the jacket in such a manner that a tightly adhered plastic coating remains on the metal component of the tape after stripping of the jacket. Yet another object of this invention is to control the level of the bond between the jacket and the shielding tape without introducing any foreign substances therebetween which are known to contribute to the degradation of either the plastic coating or the jacket or to instability of the shield/jacket bond. It is a further object of this invention to reduce the cost of cable manufacture by eliminating the need for special equipment or additional processing steps for the purpose of controlling the level of adhesion between the jacket and the shielding tape. A yet further object of this invention is to allow the cable maker a greater degree of freedom in choosing the extrusion conditions needed for high quality jacket and the optimum mechanical reinforcement of the cable through bonding of the shielding tape and the jacket. Other objects and advantages of the invention will become apparent in the following description.